Intravenous administration of the 5-HT1B receptor agonist m- chlorophenylpiperazine (m-CPP) to rats produced increases in plasma prolactin and corticosterone concentration and a decrease in plasma growth hormone. Short-term (2-4 days) or long-term (3 weeks) treatment with either the tricyclic antidepressant imipramine or the monoamine oxidase (MAO) type A inhibiting antidepressant clorgyline did not have any significant effect on the baseline levels of either of these hormones. However, long- term but not short-term imipramine treatment potentiated m- CPP's effect on plasma prolactin, while long-term but not short- term clorgyline treatment attenuated m-CPP's prolactin effect. Neither antidepressant altered m-CPP's effects on corticosterone or growth hormone. These results indicate that various agents effective in different types of affective disorders exert differential modulatory influences on serotonergic mechanisms mediating neuroendocrine responses in vivo.